I've always been sceptical of the term "portrait lens" because that implies all portraits are teh same. I've taken portraits with a 14mm lens, and sometimes with a 300mm lens. It depends what I'm trying to achieve. I can't see why animal portraits would be any different. If you are talking about classic, traditional portraits then I'd probably do the same as I would for portraits of people... a slightly long lens. On a full frame camera, around 85mm, and on a crop sensor camera, a 50mm.